In order to make available a vacuum for a pneumatic brake booster whose interior space is divided into at least one vacuum chamber and one working chamber, vacuum pumps are used which suck in residual air from the vacuum chamber and discharge it into the atmosphere. Vane-type pumps or pivoting vane pumps are generally used for this purpose in the automobile industry. Said pumps have, due to the principles involved, a large amount of friction and have to be lubricated in order to achieve an acceptable service life. Vacuum pumps which are driven by the internal combustion engine of the motor vehicle and have vanes are therefore connected to the oil circuit of the internal combustion engine. Nevertheless, an appreciable portion of the power which is output by the internal combustion engine has to be used to drive such a pump. And this is the case even when the vacuum in the chamber which is to be evacuated is already fully formed. For this reason it is appropriate to operate the vacuum pump with electrical energy and to switch it on only when the absolute pressure in the vacuum chamber rises above a predetermined value.
Furthermore, in vehicles with an electric drive or hybrid drive, the vacuum pump cannot be driven, or at certain times cannot be driven, by the internal combustion engine. For this reason, electrically driven vacuum pumps are used in these vehicles.
Equipping such an electrically driven pump with a lubricant circuit or connecting such a pump to such a circuit would mean an unacceptably high expenditure. As a result, only dry-running vacuum pumps are possible for use in motor vehicles with brake systems with an electrically driven vacuum pump. In vane-type pumps, the self-lubricating material graphite is used for this, the vanes being manufactured from graphite with the necessary precision at high cost. For this reason, efforts have been made to use a diaphragm pump for electrically supplying a braking vacuum.
A motor-pump assembly of the generic type is known, for example, from DE 10 2007 005 223 A1, wherein the outlet ducts of the motor-pump assembly are arranged in the working space covers and in the pump housing in such a way that air which is expelled from the working spaces is conducted into an interior space, surrounding the crank drive, of the pump housing, and wherein an air outlet unit is provided which permits low-noise expulsion of the air from the interior space by deflecting the air. The interior space, also referred to as the crank space, therefore serves as a sound damping space since the expelled air is not conducted directly into the atmosphere. Deflecting the air in the air outlet unit allows the noise level to be additionally significantly reduced, with the result that expulsion noises are virtually avoided completely. The air outlet unit comprises a filter housing, at least one filter, an air outlet cover, a valve holding element and a valve body, and is provided as a pre-mountable assembly. The discharged air is prevented from flowing back by virtue of the fact that the filter housing, the valve holding element and the valve body form a nonreturn valve.